The present invention relates to a powder coating composition capable of providing a matte or low gloss finish on a variety of substrates. The gloss of a cured powder coating is typically described using terms including “matte”, “low gloss”, and “mid gloss” finishes. In general, gloss is expressed as a percentage of intensity of the reflected light with respect to the intensity of the incident light at a specified angle between the incident light beam and the planar test surface. The terms “matte”, “low gloss”, and “mid gloss” are defined herein according gloss measured according to ASTM D523 at an angle of 60°. Specifically, a “matte” finish has a 60° gloss less than 20 units; a “low gloss” finish has a 60° gloss of 20 to less than 40 units; and a “mid gloss” finish has a 60° gloss of 40 to less than 60 units.
Gloss reduction in powder coatings may be achieved by producing a fine surface morphology that scatters incident light, resulting in a lower percentage of reflected light. This low percentage of reflected light gives the appearance of matte or reduced gloss. A variety of methods have been developed to provide low gloss powder coatings, including the incorporation of fillers or extenders, the incorporation of incompatible ingredients such as waxes, dry blending of different formulations, and through the incorporation of matting agents.
The incorporation of fillers or extenders has been used to produce gloss in the mid-gloss range, however this technique does not readily provide for coatings to be produced with a smooth matte or low gloss finish. Also, the incorporation of fillers can lead to coatings with reduced physical properties including impact, flexibility, and adhesion due to reduced binder concentrations.
The incorporation of incompatible ingredients such as waxes is also commonly used to produce mid gloss finishes. As with the incorporation of extenders, this technique does not readily provide for coatings with matte or low gloss. Incorporation of waxes often leads to the formation of a surface film as waxes migrate over time, and removal of the surface film can expose a glossy surface underneath.
Dry blending of two powders that have different reactivities or are immiscible has been described in, for example, U.S. Pat. No. 3,842,035 to Klaren. Dry blending requires an additional manufacturing step and the resulting dry blended components can separate over time in a conventional powder coating application and recycle process. The separation of components during the application and recycle process can yield fluctuations in gloss and curing efficiency.
The incorporation of matting agents has also been used to provide a coating with matte or low gloss appearance. The underlying process has been described as one of competing reactions or varying rates of separate reactions. The use of reactive matting agents is described in, for example, European Patent Application Nos. 72,371 A1 to Holderegger et al., and 44,030 A1 to Gude et al.; European Patent Nos. 165,207 B1 and 366,608 B1 to Lauterbach; U.S. Pat. No. 5,684,067 to Muthiah et al., U.S. Pat. No. 5,786,419 to Meier-Westhues et al.; D. H. Howell, “The Technology, Formulation and Application of Powder Coatings”, J. D. Sanders, Ed., John Wiley and Sons in association with SITA Technology Limited: London, England 2000. Vol. 1, pages 152-178; C. Grob and C. Rickert (2000) Water-Borne, Higher-Solids, and Powder Coatings Symposium, New Orleans, 1-3 Mar. 2000, pp 337-349; P. A. Chetcuti, B. Dreher, and P. Gottis, Mod Paint Coatings (1995), volume 85, no. 7, pages 28-32; J. J. Salitros and R. Patarcity, Proc. Water-Borne, Higher-Solids, Powder Coat. Symp. (1992), 19th, pages 517-526; and J. Schmidhauser and J. Havard, Proc. Int. Waterborne, High-Solids, Powder Coat. Symp. (2001), 28th, 391-404.
Techniques to lower gloss in acid functional polyester systems include the incorporation of zinc complexes as described, for example, in U.S. Pat. No. 5,684,067 to Muthiah et al., European Patent Nos. 165,207 B1 and 366,608 B1 to Lauterbach; and D. F. Loar and P. Verle, Eur. Polym. Paint Colour. J. (1994), vol. 184, issue no. 4359, pages 504-506. This route may yield inconsistent gloss when cure is carried out over a wide temperature range, and it may lead to a blooming at the coatings surface when cure is carried out at temperatures of less than 300° F.
Another technique to achieve a matte or low gloss in an acid functional polyester is to react it with a glycidyl functional acrylic polymer. This is described in, for example, D. H. Howell, cited above; T. Agawa, H. Suzuki, and H. Takeda, Poly. Mater. Sci. Eng. 1992, 204(2), pp 170-172; and U.S. Pat. No. 5,436,311 to Dumain et al. The use of glycidyl methacrylate (GMA) acrylic resins in powder coating formulations creates some disadvantages. The presence of small amounts of GMA acrylic resins may cause surface defects in powder coatings that use other types of resins and that do not contain GMA acrylic resins. As a result facilities that spray powder coatings containing GMA acrylics must isolate this powder coating spray process from spray processes that do not use GMA acrylic resins.
Examples of acid functional reactive matting agents used in epoxy systems and polyester/epoxy hybrid systems include styrene maleic anhydride copolymers or esterified styrene maleic anhydride copolymers as described in the Salitros et al. and Schmidhauser et al. references cited above. The styrene-maleic anhydride copolymers are described as only functioning as matting agents at cure temperatures of at least 400° F. in polyester/epoxy hybrid systems.
There remains a need for a coating composition that conveniently provides consistent low gloss or matte gloss over a broad temperature range.